Field of the Invention
The invention relates to a temperature sensor including two FET""s (Field Effect Transistors) that each have an operating point at which the gate voltage is essentially temperature independent for a constant drain-source current. The invention also relates to method for operating the temperature sensor.
U.S. Pat. No. 5,226,942 describes a temperature sensor that has two field-effect transistors. One of these transistors is operated in the region below the pinch-off voltage (subthreshold region) and the other is operated at an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. By comparing the voltages at the two field-effect transistors, a signal is generated that is dependent on the temperature of the transistor that is operated below the pinch-off voltage.
A temperature sensor of this type is also disclosed in U.S. Pat. No. 5,796,290. A first field-effect transistor is operated at an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. A second field-effect transistor is operated below the temperature-independent operating point and thus in a region in which, for a constant drain-source current, the gate voltage is temperature-dependent. The difference between the voltages at the first and second field-effect transistors is evaluated as a measure of the temperature.
One disadvantage of the known temperature sensors is that the change in voltage obtained because of a change in temperature is small because of the small currents and voltages in the region below the pinch-off voltage or the temperature-independent operating point. This change in voltage, therefore, can only be processed further with difficulty.
It is accordingly an object of the invention to provide a temperature sensor and a method for operating the temperature sensor which overcomes the above-mentioned disadvantages of the prior art apparatus and methods of this general type.
In particular, it is an object of the invention to provide a temperature sensor and a method for operating the temperature sensor, which for the compensation of temperature dependences in analog or digital circuits, provide an output signal that is dependent on the temperature. In this case, small changes in temperature are intended to bring about an output signal that is as large as possible.
With the foregoing and other objects in view there is provided, in accordance with the invention, a temperature sensor that includes: a first FET transistor circuit and a second FET transistor circuit that each have a gate voltage and a temperature-independent operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent. There is also a circuit for operating the first FET transistor circuit at a first operating point that is different from the temperature-independent operating point. The circuit is also provided for operating the second FET transistor circuit at a second operating point that is different from the temperature-independent operating point. The first FET transistor circuit provides a first voltage, and the second FET transistor circuit provides a first voltage. A difference between the first voltage and the second voltage is evaluated as a measure of a temperature of the first FET transistor circuit or the second FET transistor circuit.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a temperature sensor. The method includes steps of: providing the temperature sensor with a first FET transistor circuit, a second FET transistor circuit, and a circuit for operating the first FET transistor circuit and the second FET transistor circuit; evaluating a difference between a voltage at the first FET transistor circuit and a voltage at the second FET transistor circuit as a measure of a temperature at the first FET transistor circuit or the second FET transistor circuit; providing the first FET transistor circuit and the second FET transistor circuit with a temperature-independent operating point at which, for a constant drain-source current, a gate voltage is essentially temperature-independent; and operating the first FET transistor circuit and the second FET transistor circuit at a respective operating point that is different from the temperature-independent operating point.
Accordingly, the temperature sensor is distinguished by circuitry for operating the first FET transistor circuit at an operating point that lies above or below the temperature-independent operating point. The circuitry is also for operating the second FET transistor circuit at an operating point that likewise lies above or below the temperature-independent operating point. The difference between the voltages at the first and second FET transistor circuits is evaluated as a measure of the temperature at one of the FET transistor circuits.
Consequently, unlike in the case of the temperature sensors known in the prior art, in the invention the operating points of both FET transistor circuits are put outside of the temperature-independent operating point of the respective transistor. In particular, the operating point of one FET transistor circuit may be put above and the operating point of the other FET transistor circuit may be put below the temperature-independent operating point. This increases the temperature response since the two temperature responses of the two FET transistor circuits are added together. Accordingly, the temperature sensor provides a relatively large output signal even in the case of small changes in temperature.
A further advantage of the inventive solution is that the tolerance of the circuit required for setting the current is increased since the temperature-independent operating point no longer has to be found exactly in one of the FET transistor circuits.
It is pointed out that each field-effect transistor has an operating point at which, for a constant drain-source current, the gate voltage is essentially temperature-independent (zero temperature coefficient point). The drain-source current has a positive temperature coefficient below this operating point, and a negative temperature coefficient above this operating point. This is described in detail in the literature.
In one refinement of the invention, the operating point of both FET transistor circuits lies above or below the temperature-independent operating point. A relatively large tolerance of the current feed-in may be present in this variant, too.
The circuitry for operating the first FET transistor circuit and for operating the second FET transistor circuit preferably have a current generator that feeds the two transistor circuits with constant currents. In this case, the current generator advantageously increases two coupled current sources that feed the FET transistor circuits. By using a current generator that operates the two FET transistor circuits with different currents, the desired operating points of the transistor circuits can be set simply and reliably. For setting the operating points, the current generator in this case contains, for example, a current mirror that defines the operating points of the FET transistor circuits.
As an alternative, the circuit for operating the first and second FET transistor circuits in each case have a resistor connected in series with the respective FET transistor circuit. In this embodiment of variant, the FET transistor circuits are fed by the resistors.
In a development of the temperature sensor, an amplifier circuit detects the difference between the voltages at the first and second FET transistor circuits and converts the difference voltage into a control voltage, for example, for an amplifier circuit connected downstream. The amplifier circuit amplifies the difference voltage and matches it to the operating point.
The FET transistor circuits preferably in each case have at least one MOS transistor that is operated as a diode circuit, i.e. the gate is connected to the drain terminal.
In order to enable more variable generation of the difference voltage between the two FET transistor circuits, in a further refinement of the invention, cascaded MOS transistors are used for the first and/or the second FET transistor circuit, i.e. FET transistors are connected in series. In this case, the difference voltage can be set over a very wide voltage range in a simple manner by varying the supply currents and the transistor parameters.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a Temperature sensor and method for operating a temperature sensor, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.